Glial cells, commonly called neuroglia or simply glia (Greek for "glue"), are non-
neuronal cells that provide support and nutrition, maintain homeostasis, form myelin, and participate in signal transmission in the nervous system. In the human brain, glia are estimated to outnumber neurons by about 10 to 1. [http://www.sfn.org/index.cfm?pagename=brainBriefings_astrocytes sfn.org Society for Neuroscience, 2000] ]
Glial cells provide support and protection for
neurons, the other main type of cell in the nervous system. They are thus known as the "glue" of the nervous system. The four main functions of glial cells are to surround neurons and hold them in place, to supply nutrientsand oxygento neurons, to insulate one neuron from another, and to destroy pathogensand remove dead neurons. They also modulate neurotransmission. [http://www.ncbi.nlm.nih.gov/pubmed/18564180 FEBS J. 2008 Jul;275(14):3514-26.d-Amino acids in the brain: d-serine in neurotransmission and neurodegeneration.Wolosker H, Dumin E, Balan L, Foltyn VN.] ]
Function of the glial cell
Some glia function primarily as the physical support for neurons. Others regulate the internal environment of the brain, especially the fluid surrounding neurons and their synapses, and provide nutrition to nerve cells. Glia have important developmental roles, guiding migration of neurons in early development, and producing molecules that modify the growth of
axons and dendrites. Recent findings in the hippocampusand cerebellumhave indicated that glia are also active participants in synaptic transmission, regulating clearance of neurotransmitter from the synaptic cleft, releasing factors such as ATP which modulate presynaptic function, and even releasing neurotransmitters themselves.Unlike the neuron, which is generally considered permanently post-mitotic [Nature Reviews Neuroscience 8, 368-378 (May 2007) | doi|10.1038/nrn2124] , glia are capable of mitosis.
Traditionally glia had been thought to lack certain features of neurons. For example, glia were not believed to have chemical synapses or to release
neurotransmitters. They were considered to be the passive bystanders of neural transmission. However, recent studies have disproved this. For example, astrocytes are crucial in clearance of neurotransmitterfrom within the synaptic cleft, which provides distinction between arrival of action potentials and prevents toxic build up of certain neurotransmitters such as glutamate( excitotoxicity). Furthermore, at least in vitro, astrocytescan release neurotransmitter glutamate in response to certain stimulation. Another unique type of glia, the oligodendrocyte precursor cells or OPCs, have very well defined and functional synapses from at least two major groups of neurons. The only notable differences between neurons and glia, by modern scrutiny, are the ability to generate action potentials and the polarity of neurons, namely the axons and dendrites which glia lack.
It is inaccurate to consider glia as 'glue' in the nervous system as the name implies; rather, it is more of a partner to neurons. They are also crucial in the development of the nervous system and in processes such as
synaptic plasticityand synaptogenesis. Glia have a role in the regulation of repair of neurons after injury. In the CNS glia suppress repair. Astrocytesenlarge and proliferate to form a scar and produce inhibitory molecules that inhibit regrowth of a damaged or severed axon. In the PNS Schwann cells promote repair. After axon injury Schwann cells regress to an earlier developmental state to encourage regrowth of the axon. This difference between PNS and CNS raises hopes for the regeneration of nervous tissue in the CNS, for example a spinal cord injury or severance.
Types of glial cells
Microgliaare specialized macrophages capable of phagocytosisthat protect neurons of the central nervous system. They are derived from hemopoietic precursors rather than ectodermal tissue; they are commonly categorized as such because of their supportive role to neurons.
These cells comprise approximately 15% of the total cells of the central nervous system. They are found in all regions of the brain and spinal cord. Microglial cells are small relative to macroglial cells, with changing shapes and oblong nuclei. They are mobile within the brain and multiply when the brain is damaged. In the healthy central nervous system, microglia processes constantly sample all aspects of their environment (neurons, macroglia and blood vessels).
Capacity to divide
Glia retain the ability to undergo cell division in adulthood, while most neurons cannot. The view is based on the general deficiency of the mature nervous system in replacing neurons after an injury, such as a
strokeor trauma, while very often there is a profound proliferation of glia, or gliosisnear or at the site of damage. However, detailed studies found no evidence that 'mature' glia, such as astrocytes or oligodendrocytes, retain the ability of mitosis. Only the resident oligodendrocyte precursor cells seem to keep this ability after the nervous system matures. On the other hand, there are a few regions in the mature nervous system, such as the dentate gyrusof the hippocampusand the subventricular zone, where generation of new neurons can be observed.
Most glia are derived from ectodermal tissue of the developing
embryo, particularly the neural tubeand crest. The exception is microglia, which are derived from hemopoietic stem cells. In the adult, microglia are largely a self-renewing population and are distinct from macrophages and monocytes which infiltrate the injured and diseased CNS.
In the central nervous system, glia develop from the ventricular zone of the neural tube. These glia include the oligodendrocytes, ependymal cells, and astrocytes. In the peripheral nervous system, glia derive from the neural crest. These PNS glia include Schwann cells in nerves and satellite cells in ganglia.
Glia were discovered in
1856by the pathologist Rudolf Virchowin his search for a 'connective tissue' in the brain.
The human brain contains about ten times more glial cells than neurons. Following its discovery in the late 19th century, this fact underwent significant media distortion, emerging as the famous myth claiming that "we are using only 10% of our brain". The role of glial cells as managers of communications in the synapse gap, thus modifying learning pace, has been discovered only very recently (
Oligodendrocyte Section of central canal of medulla spinalis, showing ependymal and neuroglial cells. Transverse section of a cerebellar folium.
* [http://pfrieger.gmxhome.de/work/publications/pfrieger_2002.pdf Role of glia in synapse development]
* [http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T0V-4F05G3W-1&_coverDate=02%2F01%2F2005&_alid=242540802&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=4872&_sort=d&view=c&_acct=C000006078&_version=1&_urlVersion=0&_userid=75682&md5=a6462b73c196ee912e6ea3407462f0b3 article]
* [http://www.livescience.com/humanbiology/060816_neural_progenitors.html New Source of Replacement Brain Cells Found] - glial cells can transform into other cell types and reproduce indefinitely — tricks once thought exclusive to stem cells.
* [http://www.anioman.com/Profile.php?viewedArtistID=1793201250&gallery=&page=1&back=1 Artist ADSkyler] (uses concepts of neuroscience and found inspiration from Glia)
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